Name
AMD_compressed_3DC_texture
Name Strings
GL_AMD_compressed_3DC_texture
Contributors
Aaftab Munshi
Contact
Benj Lipchak, AMD (benj.lipchak 'at' amd.com)
IP Status
Please contact AMD regarding any intellectual property questions/issues
associated with this extension.
Status
Complete.
Version
Last Modified Date: February 26, 2008
Revision: 6
Number
OpenGL ES Extension #39
Dependencies
Written based on the wording of the OpenGL ES 1.1 specification.
Overview
Two compression formats are introduced:
- A compression format for two component textures. When used to store
normal vectors, the two components are commonly used with a fragment
shader that derives the third component.
- A compression format for single component textures. The single component
may be used as a luminance or an alpha value.
There are a large number of games that use luminance only and/or alpha only
textures. For example, monochrome light maps used in a few popular games
are 8-bit luminance textures. This extension describes a compression format
that provides a 2:1 compression ratio for 8-bit single channel textures.
Normal maps are special textures that are used to add detail to 3D surfaces.
They are an extension of earlier "bump map" textures, which contained per-
pixel height values and were used to create the appearance of bumpiness on
otherwise smooth surfaces. Normal maps contain more detailed surface
information, allowing them to represent much more complex shapes.
Normal mapping is one of the key features that makes the current generation
of games look so much better than earlier titles. A limitation to the
effectiveness of this technique is the size of the textures required. In an
ideal case where every surface has both a color texture map and a normal
texture map, the texture memory and bandwidth requirements would double
compared to using color maps alone.
In fact, the problem is much worse because existing block based compression
methods such as DXTc, ETC, and S3TC are ineffective at compressing normal
maps. They tend to have trouble capturing the small edges and subtle
curvature that normal maps are designed to capture, and they also introduce
unsightly block artifacts.
Because normal maps are used to capture light reflections and realistic
surface highlights, these problems are amplified relative to their impact on
color textures. The results are sufficiently poor that game artists and
developers would rather not use normal map compression at all on most
surfaces, and instead limit themselves to lower resolution maps on selected
parts of the rendered scene.
3DC provides an ideal solution to the normal map compression problem. It
provides up to 4:1 compression of normal maps, with image quality that is
virtually indistinguishable from the uncompressed version. The technique is
hardware accelerated, so the performance impact is minimal. Thus,
developers are freed to use higher resolution, more detailed normal maps,
and/or use them on all of the objects in a scene rather than just a select
few.
New Procedures and Functions
None.
New Tokens
Accepted by the parameter of CompressedTexImage2D and
CompressedTexImage3DOES:
3DC_X_AMD 0x87F9
3DC_XY_AMD 0x87FA
Additions to Chapter 3 of the OpenGL 1.3 Specification (Rasterization)
Add to Table 3.17: Specific Compressed Internal Formats
Compressed Internal Format Base Internal Format
========================== ====================
3DC_X_AMD RGB
3DC_XY_AMD RGB
Add to Section 3.8.3, Alternate Image Specification
If is 3DC_X_AMD, the compressed texture is a
single channel compressed texture. If is 3DC_XY_AMD,
the compressed textures contains two channels.
The details of these formats is not disclosed, so refer to AMD's
Compressonator tool in order to encode your textures offline:
http://ati.amd.com/developer/compressonator.html
3DC_X_AMD Format
================
This format compresses a 128 bit block into 64 bits, representing a 2:1
compression ratio. The texture lookup unit will return (x, 0, 0, 1): the
decoded X value in the red component, 0.0 in the green and blue components,
and 1.0 in the alpha component.
3DC_XY_AMD Format
=================
This format compresses a 512 bit block into 128 bits, representing a 4:1
compression ratio. The texture lookup unit will return (x, y, 0, 1): the
decoded X value in the red component, the decoded Y value in the green
component, 0.0 in the blue component, and 1.0 in the alpha component.
Using 3DC_XY_AMD to compress normal maps requires an additional step. This
is because each value in a normal map is actually a 3D vector, consisting of
3 components (x, y, z). These values must be reduced to 2-component values
in order to work with 3DC_XY_AMD. Fortunately, this can be handled in a
simple way by assuming that all of the normal vectors have a length of 1.
Given the values of two components of a vector, the value of the third
component can be found using the following mathematical relationship:
z = sqrt(1 - (x*x + y*y)). This formula can be implemented using just a
couple of fragment shader instructions.
Errors
INVALID_OPERATION is generated by TexImage2D, TexSubImage2D,
CompressedTexSubImage2D, or CopyTexSubImage2D if or
is 3DC_X_AMD or 3DC_XY_AMD.
New State
The queries for NUM_COMPRESSED_TEXTURE_FORMATS and
COMPRESSED_TEXTURE_FORMATS include 3DC_X_AMD and 3DC_XY_AMD.
Revision History
02/26/2008 Benj Lipchak Throw INVALID_OPERATION on subimage updates.
09/24/2007 Jon Leech Assign extension number.
09/05/2007 Benj Lipchak Cosmetic changes.
08/01/2007 Benj Lipchak Publication readiness.
06/02/2006 Aaftab Munshi Added IP status.
05/12/2006 Aaftab Munshi First Draft.